Power Sequencing For Embedded Flash Memory Devices

ABSTRACT

A system and method for improved power sequencing within an embedded flash memory device is disclosed.

TECHNICAL FIELD

A system and method for improved power sequencing within an embeddedflash memory device is disclosed.

BACKGROUND OF THE INVENTION

Flash memory cells using a floating gate to store charges thereon andmemory arrays of such non-volatile memory cells formed in asemiconductor substrate are well known in the art. Typically, suchfloating gate memory cells have been of the split gate type, or stackedgate type.

One prior art non-volatile memory cell 10 is shown in FIG. 1. The splitgate SuperFlash (SF) memory cell 10 comprises a semiconductor substrate1 of a first conductivity type, such as P type. The substrate 1 has asurface on which there is formed a first region 2 (also known as thesource line SL) of a second conductivity type, such as N type. A secondregion 3 (also known as the drain line) also of a second conductivitytype, such as N type, is formed on the surface of the substrate 1.Between the first region 2 and the second region 3 is a channel region4. A bit line (BL) 9 is connected to the second region 3. A word line(WL) 8 (also referred to as the select gate) is positioned above a firstportion of the channel region 4 and is insulated therefrom. The wordline 8 has little or no overlap with the second region 3. A floatinggate (FG) 5 is over another portion of the channel region 4. Thefloating gate 5 is insulated therefrom, and is adjacent to the word line8. The floating gate 5 is also adjacent to the first region 2. Acoupling gate (CG) 7 (also known as control gate) is over the floatinggate 5 and is insulated therefrom. An erase gate (EG) 6 is over thefirst region 2 and is adjacent to the floating gate 5 and the couplinggate 7 and is insulated therefrom. The erase gate 6 is also insulatedfrom the first region 2.

One exemplary operation for erase and program of prior art non-volatilememory cell 10 is as follows. The cell 10 is erased, through aFowler-Nordheim tunneling mechanism, by applying a high voltage on theerase gate EG 6 with other terminals equal to zero volt. Electronstunnel from the floating gate FG 5 into the erase gate EG 6 causing thefloating gate FG 5 to be positively charged, turning on the cell 10 in aread condition. The resulting cell erased state is known as ‘1’ state.Another embodiment for erase is by a applying a positive voltage Vegp onthe erase gate EG 6, a negative voltage Vcgn on the coupling gate CG 7,and others terminal equal to zero volts. The negative voltage Vcgncouples negatively the floating gate FG 5, hence less positive voltageVcgp is required for erasing. Electrons tunnel from the floating gate FG5 into the erase gate EG 6 causing the floating gate FG 5 to bepositively charged, turning on the cell 10 in a read condition (cellstate ‘1’). Alternately the wordline WL 8 (Vwle) and the source line SL2 (Vsle) can be negative to further reduce the positive voltage on theerase gate FG 5 needed for erase. The magnitude of negative voltage Vwleand Vsle in this case is small enough not to forward the p/n junction.The cell 10 is programmed, through a source side hot electronprogramming mechanism, by applying a high voltage on the coupling gateCG 7, a high voltage on the source line SL 2, a medium voltage on theerase gate EG 6, and a programming current on the bit line BL 9. Aportion of electrons flowing across the gap between the word line WL 8and the floating gate FG 5 acquire enough energy to inject into thefloating gate FG 5 causing the floating gate FG 5 to be negativelycharged, turning off the cell 10 in read condition. The resulting cellprogrammed state is known as ‘0’ state.

The cell 10 can be inhibited in programming (if, for instance, anothercell in its row is to be programmed but cell 10 is to not be programmed)by applying an inhibit voltage on the bit line BL 9. A split gate flashmemory operation and various circuitry are described in U.S. Pat. No.7,990,773, “Sub Volt Flash Memory System,” by Hieu Van Tran, et al, andU.S. Pat. No. 8,072,815, “Array of Non-Volatile Memory Cells IncludingEmbedded Local and Global Reference Cells and Systems,” by Hieu VanTran, et al, which are incorporated herein by reference.

FIG. 2 depicts a typical prior art architecture for a two-dimensionalprior art flash memory system. Die 12 comprises: memory array 15 andmemory array 20 for storing data, the memory array optionally utilizingmemory cell 10 as in FIG. 1; pad 35 and pad 80 for enabling electricalcommunication between the other components of die 12 and, typically,wire bonds (not shown) that in turn connect to pins (not shown) orpackage bumps that are used to access the integrated circuit fromoutside of the packaged chip or macro interface pins (not shown) forinterconnecting to other macros on a SOC (system on chip); high voltagecircuit 75 used to provide positive and negative voltage supplies forthe system; control logic 70 for providing various control functions,such as redundancy and built-in self-testing; analog circuit 65; sensingcircuits 60 and 61 used to read data from memory array 15 and memoryarray 20, respectively; row decoder circuit 45 and row decoder circuit46 used to access the row in memory array 15 and memory array 20,respectively, to be read from or written to; column decoder 55 andcolumn decoder 56 used to access the column in memory array 15 andmemory array 20, respectively, to be read from or written to; chargepump circuit 50 and charge pump circuit 51, used to provide increasedvoltages for program and erase operations for memory array 15 and memoryarray 20, respectively; high voltage driver circuit 30 shared by memoryarray 15 and memory array 20 for read and write (erase/program)operations; high voltage driver circuit 25 used by memory array 15during read and write operations and high voltage driver circuit 26 usedby memory array 20 during read and write (erase/program) operations; andbitline inhibit voltage circuit 40 and bitline inhibit voltage circuit41 used to un-select bitlines that are not intended to be programmedduring a write operation for memory array 15 and memory array 20,respectively. These functional blocks are understood by those ofordinary skill in the art, and the block layout shown in FIG. 2 is knownin the prior art.

With reference to FIG. 3, a prior art embedded flash memory system 100is depicted. Embedded flash memory system 100 comprises power managementunit 101, microcontroller unit core 102, peripherals 103 (USBx, SPI,I2C, UART, ADC, DAC, PWM, MC, HMI), SRAM 104, embedded flash device 105,and power supply bus 106. Embedded flash device 105 optionally canfollow the design of FIGS. 1 and 2, described above. Power managementunit 101 generates a plurality of voltages that are provided on powersupply bus 106. Three examples of voltages provided on power supply bus106 are VDD, VDDCORE, and VDDFLASH. VDD commonly is relatively high,such as 2.5 V, VDDCore is relatively low, such as 1.2 V, and VDDFLASH isalso relatively high, such as 2.5 V, and in some cases is equal toVDDCORE. VDDCORE often is used to power the control logic of embeddedflash memory system 100. VDD often is used to power all other functions.

With reference to FIG. 4, a typical power sequence operation is depictedfor prior art embedded flash memory system 100. During a power-upsequence, at time TU0, the voltage for Voltage 401 begins to ramp up. Attime TU1, the voltage for Voltage 402 begins to ramp up At time TU2, thevoltage for Voltage 401 begins to plateau. At time TU3, the voltage forVoltage 402 begins to plateau. Here, Voltage 401 can be VDD, and Voltage402 can be VDDFLASH.

During a power-down sequence, at time TD0, the voltage for Voltage 402begins to ramp down. At time TD1, the voltage for Voltage 401 begins toramp down. At time TD2, the voltage for Voltage 402 reaches 0 V. At timeTD3, the voltage for Voltage 401 reaches 0 V.

The prior art power sequencing of FIG. 4 can be problematic.Specifically, in the period between time TU0 and TU1, Voltage 401 mayreach a sufficient operating level while Voltage 402 is not at asufficient operating level. Similarly, in the period between time TU1and TU2, Voltage 401 may be at a sufficient operating level whileVoltage 402 is not yet at a sufficient operating level. In the timeperiod between time TD0 and TD1, Voltage 401 will still be at asufficient operating level, but Voltage 402 may fall below a sufficientoperating level. Between time TD1 and TD2, Voltage 402 will be below asufficient operating level for at least part of that period whileVoltage 401 will still be above a sufficient operating level. Betweentime TD2 and TD3, Voltage 402 will be below a sufficient operatinglevel, and Voltage 401 may still be at a sufficient operating level forat least part of the period. These inconsistencies of state for Voltage401 and Voltage 402 can cause problems in the operation of prior artembedded flash memory system 100. For example, logic circuits may not beable to operate before other circuits (such as a charge pump) are readyto operate.

SUMMARY OF THE INVENTION

What is needed is an improved power management unit to generate improvedpower sequencing for a plurality of voltage sources.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a prior art split gate flash memory cell.

FIG. 2 depicts a layout of a prior art flash memory array.

FIG. 3 depicts a prior art embedded flash memory system.

FIG. 4 depicts prior art power sequencing for two voltage sources withinan embedded flash memory system.

FIG. 5 depicts an embodiment of an embedded flash memory system.

FIG. 6 depicts a power sequencing embodiment.

FIG. 7 depicts another power sequencing embodiment.

FIG. 8 depicts another power sequencing embodiment.

FIG. 9 depicts another power sequencing embodiment.

FIG. 10 depicts another power sequencing embodiment.

FIG. 11 depicts another power sequencing embodiment.

FIG. 12 depicts a power-ready circuit.

FIG. 13 depicts a power sequence enabling circuit.

FIG. 14 depicts a voltage level shifter system.

FIG. 15 depicts a power disabling system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 5, an embodiment of an embedded flash memorysystem 500 is depicted. Embedded flash memory system 500 comprises powermanagement unit 501, microcontroller unit core 502, peripherals 503,SRAM 504, embedded flash device 505, and power supply bus 506. Embeddedflash device 505 optionally can follow the design of FIGS. 1 and 2,described above. Power management unit 501 comprises voltage source 507that generates VDD (main power supply, typically highest voltage level,typically 10 voltage level, e.g., 2.5V or 1.8V), voltage source 509 thatgenerates VDDFLASH (typically IO voltage level, e.g., 2.5V or 1.8V),voltage source 508 that generates VDDCORE (typically core logic voltagelevel, e.g.1.2V or 0.8V), and voltage source 510 that generatesVDDCOREFLASH (typically core logic voltage level, e.g.1.2V or 0.8V),each of which is provided on power supply bus 506. VDDCOREFLASH andVDDFLASH often are used to power the core logic and (mixed voltage or IOvoltage) circuits of embedded flash device 505 respectively, VDDCOREoften is used to power all other core control logic of embedded flashmemory system 500, and VDD often is used to power all other functionssuch as analog functions and IO functions. As discussed below, theembodiment of FIG. 5 follows a different power sequencing than in theprior art system of FIG. 3.

With reference to FIG. 6, power sequencing mode 600 is depicted. Voltagesource 507 generates voltage 601 (VDD) and 603 (VDDFLASH) and voltagesource 508 generates voltage 602 (VDDCORE). Here, VDD and VDDFLASH areidentical. The period between time TU0 and TU4 depict the power-on (aka,power-up) sequence, and the period between time TD0 and TD3 depicts thepower-down sequence. Unlike in the prior art, voltage 601 (VDD,VDDFLASH) and voltage 602 (VDDCORE) begin ramping up at the same time(or approximately at the same time), time TU0, during the power-upsequence, and voltage 601 (VDD, VDDFLASH) and voltage 602 (VDDCORE)reach 0V at the same time (or approximately at the same time), time TD3.In one embodiment, during the ramping up period, the voltage 602(VDDCORE) follows the voltage 601 (VDD) by an NMOS source followercircuit. In one embodiment, during the ramping down period, the voltage602 (VDDCORE) follows the voltage 601 (VDD) by a PMOS source followercircuit or a diode connection circuit (a diode connected between VDDCOREand VDD). Voltage 602 (VDDCORE) is plateaued at an intermediate levelVDDCOREINT between times TU1 and TU2 and between times TD1 and TD2. Theintermediate level VDDCOREINT is such that basic logic gates (e.g,.NAND, NOR, INV, DFF, etc.) can start to function digitally. Typicallythis level is at least equal or greater to maximum of Vtn (NMOSthreshold voltage) or Vtp (PMOS threshold voltage) value, forexample=˜0.3-0.7 Volt. Between TU3 and TD1, the voltage 602 (VDDCORE) isregulated at a final desired voltage level by a precision regulationcircuit,

With reference to FIG. 7, power sequencing mode 700 is depicted. Voltagesource 507 generates voltage 701 (VDD), voltage source 508 generatesvoltage 702 (VDDCORE), and voltage source 509 generates voltage 703(VDDFLASH). The period between time TU0 and TU5 depict the power-onsequence, and the period between time TD0 and TD2 depicts the power-downsequence. Unlike in the prior art, voltage 701 (VDD) and voltage 703(VDDFLASH) begin ramping down at the same time (or approximately at thesame time), time TD0, during the power-down sequence, and voltage 701(VDD), voltage 702 (VDDCORE), and voltage 703 (VDDFLASH) ramp down atthe same times at time TD1 (or approximately at the same time), andreach 0 at the same time, time TD2 (or approximately at the same time).During the ramping up, the voltage 702 (VDDCORE) ramps up to a finaldesired voltage at time TU3, then at some time later at TU4, the voltage703 (VDDFLASH) begins to ramp up to a final desired voltage at time TU5.In this embodiment, the voltage 702 (VDDCORE) is alive before thevoltage 703 (VDDFLASFH), meaning the voltage 702 reaching a desiredlevel first before the voltage 703 starts to ramp up. In this case, thecontrol logic of the embedded flash 505 would be able to function andhence control the chip functionality before circuits of the voltage 703(VDDFLASH) starts to function. Typically, the circuits of the voltage703 (VDFLASH) is mainly controlled by the control logic powered by thevoltage 702 (VDDCORE). In one embodiment, during the ramping up time TU0and TU4, the voltage 703 (VDDFLASH) is at a floating level (high-Z, notbeing driven).

With reference to FIG. 8, power sequencing mode 800 is depicted. Voltagesource 507 generates voltage 801 (VDD), voltage source 508 generatesvoltage 802 (VDDCORE), and voltage source 509 generates voltage 803(VDDFLASH). The period between time TU0 and TU3 depict the power-onsequence, and the period between time TD0 and TD2 depicts the power-downsequence. Unlike in the prior art, voltage 801 (VDD) and voltage 803(VDDFLASH) begin ramping up at the same time, time TU0, during thepower-up sequence, and voltage 801 (VDD), voltage 802 (VDDCORE), andvoltage 803 (VDDFLASH) reach 0V at the same time, time TD2. During thepower-up phase, while the voltage 801 and 803 (VDDFLASH) are ramping upand stabilize at a final voltage between time TU0 and TU1, the voltage802 (VDDCORE) essentially stays at zero volts and starts to ramp up attime TU2 and stabilize at time TU3. During time TU0 and TU3 the circuitsof voltage 803 (VDDFLASH) and the circuits of the voltage 802 (VDDCORE)are enabled or disabled by a VDD control logic powered by the voltage801 (VDD). In one embodiment, the voltage 802 (VDDCORE) is at a floatlevel (high-Z) during the voltage 801 (VDD) ramping up period betweenTU0 and TU1.

With reference to FIG. 9, power sequencing mode 900 is depicted. Voltagesource 507 generates voltage 901 (VDD), voltage source 508 generatesvoltage 902 (VDDCORE), and voltage source 509 generates voltage 903(VDDFLASH). The period between time TU0 and TU5 depict the power-onsequence, and the period between time TD0 and TD4 depicts the power-downsequence. The power-up sequence is similar to that of the power upsequence mode 700. The power-down sequence a mirrored sequence of thepower up sequence.

With reference to FIG. 10, power sequencing mode 1000 is depicted.Voltage source 507 generates voltage 1001 (VDD), voltage source 508generates voltage 1002 (VDDCORE), voltage source 509 generates voltage1003 (VDDFLASH), and voltage source 510 generates voltage 1004(VDDCOREFLASH). The period between time TU0 and TU4 depicts the power-onsequence, and the period between time TD0 and TD4 depicts the power-downsequence. Unlike in the prior art, voltage 1001 (VDD) and voltage 1002(VDDCORE) reach 0V at the same time, time TD4. The voltage 1003(VDDFLASH) and the voltage 1004 (VDDCOREFLASF) and/or the voltage 1001(VDD) and/or the voltage 1002 (VDDCORE) are supplied to the embeddedflash device 505. The voltage 1003 (VDDFLASH) and the voltage 1004(VDDCOREFLASF) both are ramping up and down at the same time (orapproximately at the same time).

In the above described power sequence modes 600, 700, 800, 900, 1000,and 1100, the embedded flash device 505 receives the voltage603/703/803/903/1003/1103 (VDDFLASH), the voltage 1004 (VDDCOREFLASH),and/or the voltage 601/701/801/901/1001/1101 (VDD) and/or the voltage602/702/802/902/1002/1102 (VDDCORE). In one embodiment, high voltagecharge pump circuits needed for flash such as for programming anderasing is powered from the voltage 601/701/801/901/1001/1101 VDD. Inanother embodiment, high voltage charge pump circuits needed for flashsuch as for programming and erasing is powered from the voltage603/703/803/903/1003/1103 VDDFLASH.

With reference to FIG. 11, power sequencing mode 1100 is depicted.Voltage source 507 generates voltage 1101 (VDD), voltage source 508generates voltage 1102 (VDDCORE), and voltage source 509 generatesvoltage 1103 (VDDFLASH). The period between time TU0 and TU4 depicts thepower-on sequence, and the period between time TD0 and TD4 depicts thepower-down sequence. Unlike in the prior art, voltage 1102 (VDDCORE) andvoltage 1103 (VDDFLASH) begin ramping up at the same time, time TU2,during the power-on sequence and begin ramping down at the same time,time TD3, during the power-down sequence, and voltage 1101 (VDD),voltage 1102 (VDDCORE), and voltage 1103 (VDDFLASH) reach 0V at the sametime, time TD2. The power-down sequence is mirrored sequence of thepower up sequence. The voltage 1103 (VDDFLASH) starts to ramp up afterthe voltage 1102 (VDDCORE) stabilizes.

With reference to FIG. 12, control system 1200 for power management unit501 is depicted. Reset signal 1210 is coupled to voltage source 507,voltage source 508, voltage source 509, and voltage source 510. Whenreset signal 1210 is active, voltage source 507, voltage source 508,voltage source 509, and voltage source 510 are reset, which can compriseentering power-down mode. Detector circuit 1207 receives a voltage (VDD)from voltage source 507, detector circuit 1208 receives a voltage(VDDCORE) from voltage source 508, detector circuit 1209 receives avoltage (VDDFLASH) from voltage source 509, and detector circuit 1210receives a voltage (VDDCOREFLASH) from voltage source 510.

Detector circuit 1207 determines if the voltage from voltage source 507is above thresholds V1A and V1B (V1B>V1A), and outputs detection readysignals 1217A and 1217B respectively. If signal 1217A/1217B is high,then the voltage from voltage source 507 (VDD) is above the thresholdV1A/V1B.

Detector circuit 1208 determines if the voltage from voltage source 508is above thresholds V2 A and V2B (V2B>V2A), and outputs detection readysignals 1218 A and 1218B respectively. If signal 1218A/1218B is high,then the voltage from voltage source 508 (VDDCORE) is above thethreshold V2A/V2B.

Detector circuit 1209 determines if the voltage from voltage source 509is above thresholds V3 A and V3B (V3B>V3A), and outputs detection readysignal 1219 A and 1219B respectively. If signal 1219A/1219B is high,then the voltage from voltage source 509 (VDDFLASH) is above thethreshold V3A/V3B.

Detector circuit 1210 determines if the voltage from voltage source 510is above thresholds V4 A and V4B (V4B>V4A), and outputs detection readysignal 1220 A and 1220B respectively. If signal 1220A/1220B is high,then the voltage from voltage source 510 (VDDCOREFLASH) is above thethreshold V4A/V4B.

The signals 1217A/1217B, 1218A/1218B, 1219A/1219B,1220A/1220B are usedto control circuits and chip functionality during ramping up of thepower up sequence and during ramping down of the power down sequencesuch as to avoid circuit contention and undesirable power consumption.

With reference to FIG. 13, power sequence enabling circuitry 1300 isdepicted. Power sequence enabling circuitry 1300 comprises PMOStransistor 1301, NMOS transistor 1302, NMOS transistor 1303, andoptional NMOS transistor 1304, coupled as shown in FIG. 13. VDDFLASH1305 is generated when VDD 1306 is present and ENVDDFLASH_N 1307 isactive. VDDFLASH 1305 will be less than VDD 1306 based on the voltagedrop across PMOS transistor 1301. When ENVDDFLASH_N 1307 is not active,VDDFLASH 1305 will drop to a lesser voltage determined by VDDFLASH-BIAS1308 and the voltage drop across NMOS transistor 1302. Thus, VDDFLASH1305 will drop to a voltage around VDDFLASH-BIAS 1308 instead of to 0 V.In another embodiment the VDDFLASH-BIAS 1308 is equal to core logicpower supply VDD such as the voltage 702 (VDDCORE) of the powersequencing mode 700. In another embodiment the VDDFLASH-BIAS 1308 isfloating (high-Z). In another embodiment the transistors 1302, 1303, and1304 are not connected, meaning the VDDFLASH 1305 is floating (high-Z)when the PMOS transistor 1301 is not enabled.

With reference to FIG. 14, VDD level shifter system 1400 is depicted.VDD level shifter system 1400 comprises PMOS transistor 1401 coupled toNMOS transistor 1402 as shown. VDD level shifter system 1400 furthercomprises NMOS transistor 1403, PMOS transistor 1404, NMOS transistor1405, PMOS transistor 1406, NMOS transistor 1407, PMOS transistor 1408,NMOS transistor 1409, PMOS transistor 1410, and NMOS transistor 1411,coupled as shown. The PMOS transistor 1410 and NMOS transistor 1411 arepowered by a core logic power supply 1414 (VDDCORE). The PMOS transistor1401 and NMOS transistor 1402 are powered by an IO power supply 1415(VDD). The PMOS transistors 1404/1406 and NMOS transistors 1405/1407 arepowered by the IO power supply 1415 (VDD). The transistors1401,1402,1403,1409 and 1408 constitutes power controlling element forthe VDD level shifter 1400. The transistors 1404, 1406, 1405, 1407,1410, and 1411 constitutes a normal level shifter. When DIS_VDD 1412 isset to “1,” OUT_VDD 1413 will be VDD, and OUTB_VDD 1414 will be 0. Inone embodiment, power supply level for transistor 1401 and the signalDIS_VDD 1412 is greater or equal to power supply level for thetransistors 1404, 1406, and 1408. With this circuit configuration theoutputs 1414 and 1413 are at known state with control signal DIS_VDD1412 being active.

With reference to FIG. 15, power disabling system 1500 is depicted.Power disabling system 1500 comprises PMOS transistor 1501 and NMOStransistor 1502, coupled as shown. Power disabling system 1500 furthercomprises PMOS transistor 1503, NMOS transistor 1504, NMOS transistor1505, NMOS transistor 1506, PMOS transistor 1507, NMOS transistor 1508,PMOS transistor 1509, NMOS transistor 1510, and PMOS transistor 1511,coupled as shown as a power supply level shifter. The transistor 1501,1502, 1504, 1506, and 1511 will cause the outputs of this level shifterto be at a known state similar to that of the circuit 1400 due tosimilar power controlling elements. Power disabling system 1500 furthercomprises PMOS transistor 1512 (its bulk connected to its source), PMOStransistor 1513 (its bulk connected to its drain), PMOS transistor 1514(its bulk connected to its source), and PMOS transistor 1515 (its bulkconnected to its drain), coupled as shown. When DIS_VDD 1516 is “1,”VDDxVDDCORE 1517 is equal to VDDCORE.

References to the present invention herein are not intended to limit thescope of any claim or claim term, but instead merely make reference toone or more features that may be covered by one or more of the claims.Materials, processes and numerical examples described above areexemplary only, and should not be deemed to limit the claims. It shouldbe noted that, as used herein, the terms “over” and “on” bothinclusively include “directly on” (no intermediate materials, elementsor space disposed there between) and “indirectly on” (intermediatematerials, elements or space disposed there between). Likewise, the term“adjacent” includes “directly adjacent” (no intermediate materials,elements or space disposed there between) and “indirectly adjacent”(intermediate materials, elements or space disposed there between). Forexample, forming an element “over a substrate” can include forming theelement directly on the substrate with no intermediatematerials/elements there between, as well as forming the elementindirectly on the substrate with one or more intermediatematerials/elements there between.

What is claimed is:
 1. A power management unit comprising a firstvoltage source and a second voltage source configured to perform apower-on sequence, wherein: during a first time period, the voltageoutput from the first voltage source ramps upward and the voltage outputfrom the second voltage sour ramps upward; during a second time periodimmediately following the first time period, the voltage output from thefirst voltage source ramps upward and the voltage output from the secondvoltage remains at a constant level; during a third time periodimmediately following the second time period, the voltage output fromthe first voltage source ramps upward and the voltage output from thesecond voltage sour ramps upward; and during a fourth time periodimmediately following the third time period, the voltage output from thefirst voltage source ramps upward and the voltage output from the secondvoltage source remains at a constant level.
 2. The power management unitof claim 1, wherein the first voltage source and the second voltagesource are configured to perform a power-down sequence, wherein: duringa fifth time period, the voltage output from the first voltage sourceramps downward and the voltage output from the second voltage remains ata constant level; during a sixth time period immediately following thefifth time period, the voltage output from the first voltage sourceramps downward and the voltage output from the second voltage rampsdownward and then remains at a constant level; and during a seventh timeperiod immediately following the sixth time period, the voltage outputfrom the first voltage source ramps downward and the voltage output fromthe second voltage source ramps downward.
 3. A power management unitcomprising a first voltage source, a second voltage source, and a thirdvoltage source configured to perform a power-on sequence, wherein:during a first time period, the voltage output from the first voltagesource ramps upward, the voltage output from the second voltage sourremains at a constant level, and the voltage output from the thirdvoltage remains at a constant level; during a second time periodimmediately following the first time period, the voltage output from thefirst voltage source remains at a constant level, the voltage outputfrom the second voltage remains at a constant level, and the thirdvoltage source remains at a constant level; during a third time periodimmediately following the second time period, the voltage output fromthe first voltage source remains at a constant level, the voltage outputfrom the second voltage source ramps upward, and the voltage output fromthe third voltage sour remains at a constant level; during a fourth timeperiod immediately following the third time period, the voltage outputfrom the first voltage source remains at a constant level, the voltageoutput from the second voltage remains at a constant level, and thethird voltage source remains at a constant level; and during a fifthtime period immediately following the fourth time period, the voltageoutput from the first voltage source remains at a constant level, thevoltage output from the second voltage source remains at a constantlevel, and the voltage output from the third voltage source rampsupward.
 4. The power management unit of claim 3, wherein the firstvoltage source, the second voltage source, and the second voltage sourceare configured to perform a power-down sequence, wherein: during a sixthtime period, the voltage output from the first voltage source rampsdownward, the voltage output from the second voltage remains at aconstant level, and the voltage output from the third voltage sourceramps downward; during a seventh time period immediately following thesixth time period, the voltage output from the first voltage sourceramps downward, the voltage output from the second voltage source rampsdownward, and the voltage output from the third voltage ramps downward.5. The power management unit of claim 3, wherein the first voltagesource, the second voltage source, and the second voltage source areconfigured to perform a power-down sequence, wherein: during a sixthtime period, the voltage output from the first voltage source remains ata constant level, the voltage output from the second voltage remains ata constant level, and the voltage output from the third voltage sourceramps downward; during a seventh time period immediately following thesixth time period, the voltage output from the first voltage sourceremains at a constant level, the voltage output from the second voltagesource remains at a constant level, and the voltage output from thethird voltage remains at a constant level. during an eighth time periodimmediately following the seventh time period, the voltage output fromthe first voltage source ramps downward, the voltage output from thesecond voltage source remains at a constant level, and the voltageoutput from the third voltage remains at a constant level; and during aninth time period immediately following the eighth time period, thevoltage output from the first voltage source ramps downward, the voltageoutput from the second voltage source remains at a constant level, andthe voltage output from the third voltage ramps downward.
 6. A powermanagement unit comprising a first voltage source, a second voltagesource, and a third voltage source configured to perform a power-onsequence, wherein: during a first time period, the voltage output fromthe first voltage source ramps upward, the voltage output from thesecond voltage sour remains at a constant level, and the voltage outputfrom the third voltage ramps upward; during a second time periodimmediately following the first time period, the voltage output from thefirst voltage source remains at a constant level, the voltage outputfrom the second voltage remains at a constant level, and the thirdvoltage source remains at a constant level; during a third time periodimmediately following the second time period, the voltage output fromthe first voltage source remains at a constant level, the voltage outputfrom the second voltage source ramps upward, and the voltage output fromthe third voltage sour remains at a constant level;
 7. The powermanagement unit of claim 6, wherein the first voltage source, the secondvoltage source, and the second voltage source are configured to performa power-down sequence, wherein: during a fourth time period, the voltageoutput from the first voltage source ramps downward, the voltage outputfrom the second voltage remains at a constant level, and the voltageoutput from the third voltage source ramps downward; during a fifth timeperiod immediately following the fourth time period, the voltage outputfrom the first voltage source ramps downward, the voltage output fromthe second voltage source ramps downward, and the voltage output fromthe third voltage ramps downward.
 8. A power management unit comprisinga first voltage source, a second voltage source, a third voltage source,and a fourth voltage source configured to perform a power-on sequence,wherein: during a first time period, the voltage output from the firstvoltage source ramps upward, the voltage output from the second voltagesour remains at a constant level, the voltage output from the thirdvoltage remains at a constant level, and the voltage output from afourth voltage remains at a constant level; during a second time periodimmediately following the first time period, the voltage output from thefirst voltage source remains at a constant level, the voltage outputfrom the second voltage remains at a constant level, the voltage outputfrom the third voltage source remains at a constant level, and thevoltage output from the fourth voltage source remains at a constantlevel; during a third time period immediately following the second timeperiod, the voltage output from the first voltage source remains at aconstant level, the voltage output from the second voltage ramps upwardand then remains at a constant level, the voltage output from the thirdvoltage source remains at a constant level, and the voltage output fromthe fourth voltage source remains at a constant level; and during afourth time period immediately following the third time period, thevoltage output from the first voltage source remains at a constantlevel, the voltage output from the second voltage remains at a constantlevel, the voltage output from the third voltage source ramps upward,and the voltage output from the fourth voltage source ramps upward 9.The power management unit of claim 8, wherein the first voltage source,the second voltage source, the third voltage source, and the fourthvoltage source are configured to perform a power-down sequence, wherein:during a fifth time period, the voltage output from the first voltagesource remains at a constant level, the voltage output from the secondvoltage remains at a constant level, the voltage output from the thirdvoltage source ramps downward, and the voltage output from the fourthvoltage source remains constant and then ramps downward; during a sixthtime period immediately following the fifth time period, the voltageoutput from the first voltage source remains at a constant level, thevoltage output from the second voltage remains at a constant level, thevoltage output from the third voltage source remains at a constantlevel, and the voltage output from the fourth voltage source remains ata constant level; during a seventh time period immediately following thesixth time period, the voltage output from the first voltage sourceramps downward, the voltage output from the second voltage remains at aconstant level, the voltage output from the third voltage source remainsat a constant level, and the voltage output from the fourth voltagesource remains at a constant level; and during an eighth time periodimmediately following the seventh time period, the voltage output fromthe first voltage source ramps downward, the voltage output from thesecond voltage ramps downward, the voltage output from the third voltagesource remains at a constant level, and the voltage output from thefourth voltage source remains at a constant level.
 10. A powermanagement unit comprising a first voltage source, a second voltagesource, and a third voltage source configured to perform a power-onsequence, wherein: during a first time period, the voltage output fromthe first voltage source ramps upward, the voltage output from thesecond voltage sour remains at a constant level, and the voltage outputfrom the third voltage ramps remains at a constant level; during asecond time period immediately following the first time period, thevoltage output from the first voltage source remains at a constantlevel, the voltage output from the second voltage remains at a constantlevel, and the third voltage source remains at a constant level; duringa third time period immediately following the second time period, thevoltage output from the first voltage source remains at a constantlevel, the voltage output from the second voltage source ramps upwardand then remains at a constant level, and the voltage output from thethird voltage source ramps upward and then remains at a constant level;and during a fourth time period immediately following the fifth timeperiod, the voltage output from the first voltage source remains at aconstant level, the voltage output from the second voltage source rampsupward and then remains at a constant level, and the voltage output fromthe third voltage source ramps upward.
 11. The power management unit ofclaim 10, wherein the first voltage source, the second voltage source,and the second voltage source are configured to perform a power-downsequence, wherein: during a fifth time period, the voltage output fromthe first voltage source remains at a constant level, the voltage outputfrom the second voltage remains at a constant level, and the voltageoutput from the third voltage source ramps downward; during a sixth timeperiod immediately following the fifth time period, the voltage outputfrom the first voltage source remains at a constant level, the voltageoutput from the second voltage source remains at a constant level, andthe voltage output from the third voltage remains at a constant level;during a seventh time period immediately following the sixth timeperiod, the voltage output from the first voltage source ramps downward,the voltage output from the second voltage source remains at a constantlevel, and the voltage output from the third voltage remains at aconstant level; and during a eighth time period immediately followingthe seventh time period, the voltage output from the first voltagesource remains ramps downward, the voltage output from the secondvoltage source ramps downward, and the voltage output from the thirdvoltage ramps downward.
 12. A method of operating a power managementcontrol system comprising a main power source, a core logic powersource, and a power control unit, comprising: providing a firstdetection ready output signal when an output voltage from the main powersource passes a first predetermined level; providing a second detectionready output signal when an output voltage from the core logic powersource passes a second predetermined level; enabling, by the powercontrol unit in response to the first detection ready output signal, thesupply of power from the main power source to an embedded flash memorydevice; and enabling, by the power control unit in response to thesecond detection ready output signal, the supply of power from the corelogic power source to the embedded flash memory device.
 13. The methodof claim 12, further comprising: enabling, by the power control unit,the supply of power from the main power source to a charge pump circuit.14. A power sequence enabling circuit comprising a PMOS transistor; afirst NMOS transistor; a first voltage source, configured to perform apower-on sequence, wherein: during a power up time period, a voltageoutput from the first voltage source ramps upward, toward a voltageoutput from a second voltage source through the PMOS transistor, andduring a power down period, a voltage from the second voltage sourceramps downward toward an intermediate voltage greater than zero voltsthrough the first NMOS transistor.
 15. The circuit of claim 14, furthercomprising: a power management control system to control enablingcircuitry for the first voltage source and second voltage source. 16.The circuit for claim 15, wherein the power management control systemprovides a plurality of detection ready output signals to control theenabling circuitry.
 17. The circuit of claim 14, wherein theintermediate voltage is floating.
 18. A power disabling systemcomprising: a first set comprising a first PMOS transistor and a secondPMOS transistor, wherein the bulk of the first PMOS transistor isconnected to the source of the first PMOS transistor, the bulk of thesecond PMOS transistor is connected to the drain of the second PMOStransistor, and the drain of the first PMOS transistor is connected tothe source of the second PMOS transistor; a second set comprising athird PMOS transistor and a fourth PMOS transistor, wherein the bulk ofthe third PMOS transistor is connected to the source of the third PMOStransistor, the bulk of the fourth PMOS transistor is connected to thedrain of the fourth PMOS transistor, and the drain of the third PMOStransistor is connected to the source of the fourth PMOS transistor;wherein the drain of the second PMOS transistor is connected to thedrain of the fourth PMOS transistor; a first voltage source connected tothe source of the first PMOS transistor; a second voltage sourceconnected to the source of the third PMOS transistor; and a power levelshifting circuit with known state outputs that control the first set andthe second set.